After graduating from Harvard, Harrop unsuccessfully looked for editorial work with various newspapers.[1] At the invitation of Frank Boyden, the headmaster of Deerfield Academy, Harrop spent about five months teaching at the school before entering the Marine Corps, in which he served during the Korean War.[1]

After returning to the United States, Harrop took and passed the Foreign Service exam in 1952.[1] At the time, Harrop was still interested in journalism, and used the G.I. Bill to earn a master's degree in journalism from the University of Missouri School of Journalism in Columbia, Missouri.[1] Harrop then accepted an appointment to the United States Foreign Service; according to an oral history given by Harrop, he "was one of a considerable group of people who were delayed in entering the Foreign Service for a couple of years by Senator Joseph McCarthy."[1] According to Harrop, "Accepting the appointment was a difficult decision for me. I had heard nothing from the Department for months, and was suddenly told over the phone that if I would appear within nine days in Washington I would receive an appointment and would go to Palermo."[1]

Harrop entered the Foreign Service in a year when the new class of diplomats did not attend the standard A-100 orientation and training course at the Foreign Service Institute.[1] One of Harrop's sons was born the day before he left for Italy for his first assignment, at the U.S. consulate general in Palermo, Sicily in 1953.[1] This consulate would have ordinarily had only twelve staffers, but because of the Refugee Relief Act had exploded to at least eighty staffers.[1] Harrop was one of several vice consuls there.[1] Harrop helped administer the Refugee Relief Program on the island, where at the time the Mafia held great sway.[1]

Harrop served in Palermo from 1954 to October 1955, when he was transferred to the U.S. Embassy in Rome to be assistant commercial attaché there under Ambassador Clare Boothe Luce and, later, James David Zellerbach.[1] Harrop left Rome to return to Washington in the late fall of 1958.[1] According to Harrop's oral history years later, "I came back under protest because by that time I had become fascinated with energy questions and with the politics of energy -- atomic energy particularly. I was asked to come back to be in the Office of Personnel in the State Department, handling personnel assignments. I sent back an imprudent cable to say that if I had wanted to be in personnel work I would have joined Westinghouse, not the State Department. ... I was told in no uncertain terms that if I wanted to continue my career in the State Department, I should come back and work in Personnel. So I did."[1] At the Office of Personnel, Harrop was initially a placement officer and later deputy director of the Office of Washington Assignments.[1] Around 1961, Harrop left Personnel and moved to the regional affairs office of the Bureau of European Affairs, where he replaced Arthur A. Hartman as a desk officer focused on EURATOM.[1]

Harrop returned in Washington, D.C. to become director of Regional African Affairs at the State Department's Bureau of Intelligence Research, serving there from 1969 to 1971, under Ray S. Cline, then the assistant secretary for intelligence and research.[1] Harrop also became involved in State Department personnel matters; he was elected to a position on the board of the American Foreign Service Association, and later became chairman, taking a yearlong leave without pay from the State Department in the year 1971.[1] Harrop then returned to the Department of State in 1972-1973 to join the Policy Planning Council.[1] Harrop left Policy Planning when he was chosen by U.S. Ambassador to AustraliaMarshall Green to serve as deputy chief of mission (DCM) in Canberra.[1] Harrop served as DCM in Australia from 1973 to 1975.[1]

Harrop returned to the U.S. and was on home leave when he was appointed U.S. Ambassador to Guinea.[1] Harrop served in this post from May 29, 1975 to July 15, 1977.[2] As ambassador in Conakry, Guinea, Harrop faced hostility from President Ahmed Sékou Touré, leader of a radical, pan-Africanist movement.[1] Harrop's twelve-member mission was also dramatically outnumbered by the Soviet mission, which had 1200 people, and the Chinese mission, which had 700 people.[1] Key issues handled during Harrop's tenure in Conakry were Soviet overflights and bases in the country and U.S. humanitarian aid in Guinea.[1] In 1977, Harrop left Guinea and returned to the Bureau of African Affairs to become principle deputy to Assistant Secretary of State for African AffairsRichard M. Moose.[1] At the Bureau of African Affairs, Harrop spent most time of issues relating to Zaire and the Horn of Africa (particularly the Ogaden War between Ethiopia and Somalia), while Moose handled most matters relating to southern Africa (South Africa, Rhodesia, Namibia, and Angola).[1]

Harrop left the Bureau of African Affairs in 1980[1] and became U.S. Ambassador to Kenya, serving from July 10, 1980 to September 1, 1983.[2] (Harrop concurrently served as U.S. Ambassador to the Seychelles, from August 26, 1980 to September 22, 1983).[2] Kenya was then led by President Daniel arap Moi. Key issues dealt with by Harrop in this position include the application of the Foreign Corrupt Practices Act to U.S. commercial ventures in Kenya; the use of Kenyan coasts by U.S. Navy ships; territorial disputes between Kenya and Somalia, Ethiopia, Uganda, and Tanzania; and human rights in the country.[1]

Harrop returned to Washington to become Inspector General of the Department of State and the Foreign Service, serving from December 12, 1983, to August 27, 1986.[1][2] Harrop was part of a group of three diplomats appointed by Secretary of State George P. Shultz as part of a new "management team," the others being Ronald I. Spiers (Under Secretary for Management) and Alfred (Roy) Atherton (Director General of the Foreign Service).[1] As inspector general, Harrop reformed the office's investigative and auditing functions and oversaw investigations relating to allegations of corruption and sexual harassment.[1] Harrop oversaw investigations related to Faith Whittlesey, ambassador to Switzerland.[1] He also clashee with Senator Jesse Helms, who Harrop stated in a later oral history was "determined to destroy the institution of the Inspector General of the Foreign Service" and carried out a "relentless campaign against career Foreign Service professionals."[1]

After returning to the United States, Harrop was informed that he was one of the candidates for Pakistan or Israel.[1] Eventually, Harrop was appointed ambassador to Israel, his final posting; Harrop succeeded William Andreas Brown in the post.[1] He served from January 21, 1992 to May 7, 1993.[2] Important events during Harrop's tenure included the Dotan affair, the recognition of the PLO by Israel for the first time, and the beginning of the talks that led to the Oslo I Accord.[1]

Harrop is a member of the Washington Institute of Foreign Affairs and a director of the American Academy of Diplomacy, the Senior Living Foundation of the American Foreign Service, American Diplomacy Publishers, the Henry L. Stimson Center, and the Washington Humane Society.[3] Harrop is also chair of the Foreign Affairs Museum Council, which aims to establish a museum of U.S. diplomacy.[3]